Because that is what you're potentially choosing every time you choose a man.

So ask yourself, would you be happy for your son to turn out exactly like this man? Would you be proud of him? Would you be disappointed to find out he's treating a girlfriend the way this guy is treating you? Would you be happy for this man to model for your daughter what to expect in a relationship? If she brought home someone like him, would you be delighted for her or worried? In the event that the relationship doesn't work out because sometimes they just don't, do you trust that this man could stay civil and cooperative and still be a good role model even if you weren't together anymore?

Choose the father of your children very wisely, because we've all seen how much multigenerational misery a bad choice can cause. Many of us have experienced it first hand, and ironically it made us less well equipped to make good choices for ourselves. But it can be easier to do what's best for someone other than ourselves, especially when we're still working on breaking out of that conditioning, and when there is no one else thinking far, far into the future can be helpful. Even for the ladies who've decided not to have kids, this is a good thought experiment.

*A lot of people reading this are interpreting it as being a good father is the main thing you should be looking for in a man. That's not the point I was trying to get at, though it is something important to consider because statistically most of us will end up having children eventually. I'm trying to encourage you to step back and evaluate a man from another perspective, ie. if this was someone you cared deeply about in a non-romantic relationship, would you be disappointed by their behaviour? Feel free to substitute child with sister, brother, or best friend if you can't relate to the idea of having kids some day, though I still think the kid argument has the most impact because children do become what they see every day and they will grow up to have bad relationships if you don't model a healthy one for them.